1. Field of the Invention
This invention relates to a nut which has a female thread for fastening another member to a metallic plate by a bolt, a female thread machining device and a female thread machining method for forming the female thread.
2. Description of Related Art
Japanese Patent Unexamined Publication JP-A-8-200343 discloses that a bolt is fastened to a nut which is integrated with a washer to fix two metallic plates which are fastened members. In this case, the nut is not integrated with the metallic plates, and thus, for fastening the nut to the metallic plate, drilling process on the metallic plates and provisional fixing of the nut to the bore are required.
In the nut of JP-A-8-200343, a seat surface is convex shape so that a side of fastening component is higher toward its center. Thus, a workpiece is difficult to be injured on the seat surface of the nut. Further, this nut has a shape in which radial thickness becomes thinner toward a not-fastening-component side, however, the operation and effect owing to this structure are not particularly described in the JP-A-8-200343.
On the other hand, in manufacturing a vehicle, when welding the nut on a bolting point such as a door hinge of a frame, precise positioning in welding process is difficult and also the welding process is troublesome.
Japanese Patent Unexamined Publication JP-A-1-306124 proposes a device capable of forming various female threads on a workpiece. However, this device is not suitable to form the female thread on a thin plate such as a frame of a motor vehicle.
Japanese Patent Unexamined Publication JP-A-2005-329528 proposes a female machining device which does not require welding process of the nut. In this machining device, a machining tool is inserted into a metallic plate while rotating the machining tool and a hole is formed on the metal plate by a small diameter section of the machining tool. Then, while inserting the machining tool, the machining tool is rotated and inserted further and a female thread is formed in the hole by a tapping section of the machining tool. Further, by adding excessive material onto the hole as a filler, this machining device can be also applied to a thin plate such as the frame of the motor vehicle. As a result, a bushing having the female thread can be formed by a single process.
Meanwhile, where the nut is welded on the metallic plate, the nut essentially has a sufficient strength and has a sufficient fastening strength for the bolt. However, the female thread formed on the bushing formed by adding the excessive material onto the thin plate does not always have a sufficient strength. Thus, careful experiment and inspection therefor are required.
Further, where the nut formed monolithically with the metallic plate is applied to the motor vehicle, vibration will be applied to the nut for a long time. Thus, there is a fear of loosing the fastening of bolt and the fastening torque must be particularly high, or otherwise some locking means must be provided.
Further, Japanese Patent Unexamined Publication JP-A-2006-118582 discloses a concave nut in which a seat surface on a component fastening side is concaved toward the center and discloses that torque is applied until the concave becomes deformed to be brought into plane-contact with the workpiece. According to this nut, great fastening torque is needed to deform the concave so that loosening is not likely to occur.
“NEJI TEIKETU, ‘SIN’ JOUSIKI NO USO, NEJI NO OUGI WO ZENKOUKAI” (edited by Nikkei Mechanical, first edition, second printing, Nikkei BP Co. Ltd. May 9, 1997, pages 77 to 79), points out that in fastening the bolt and nut, a first crest and a first root in the component fastening side intensively generate fastening force whereas the fastening force bore by a second crest and a second root and later is significantly small.
The nut described in the above JP-A-2006-118582 is very thick radial thickness, and so requires a considerably high torque to deform the concave on the component fastening side. Further, in this nut, the concave on the component fastening side functions as a spring; this is not so different from inserting a spring washer.
By the way, as pointed out by the above described non-Patent-Literature, the torque applied to the bolt/nut is mainly bore by the first crest and first root. Thus, simply enlarging the torque, it does not lead that the torque bored by the second and later crest and root becomes large. Namely, even if enlarging the fastening torque, the thread of the second crest and second root and later do not sufficiently contribute to the fastening force. Thus, a more efficiently locking structure has been demanded.